Detection of chemical and nuclear proliferation activities is a key issue in arms control treaty verification. There is a great need for remote sensing techniques that can determine airborne effluent signatures of a processing plant. Remote millimeter-wave spectroscopy offers great potential for this application because the millimeter-waves can penetrate through the atmosphere and can operate under all-weather conditions, including day or night times.
The Clean Air Act Amendments of 1990 dictate many new regulations, of which one that will have a significant impact on process industries is the requirement for continuous emissions monitoring devices. The availability of chemical sensors for stack gas or effluent monitoring is very limited. Most are species-specific and point-type sensors. In addition to in-stack monitoring, there is a need for standoff, wide-area monitoring of chemicals that are released from stacks and vents of process and waste treatment plants. Such a sensor will not only provide a demonstration of environmental compliance, but also will serve as an indicator of impending runaway reactions and of inefficient process conditions.
Fourier-transform infrared (FTIR) techniques are currently being explored for stack-gas monitoring. Laboratory FTIR instruments are being modified for field applications. Compared to infrared techniques that mainly show vibrational bands of molecular functional groups, the millimeter-wave spectra are generally unique to a molecule and often simpler to interpret because there are no complications with respect to vibrational or electronic transitions. There are no moving parts as in FTIR, so the millimeter-wave sensor is less affected by floor vibrations and maintenance is easier. Millimeter-wave techniques can be used in smoky and dusty environments. Millimeter-waves can transit thick dielectric walls and particulate laden gases. Optical techniques require transparent windows which must be cleaned or purged periodically. Also, millimeter radiation is less affected by dust and cloud conditions, and can provide longer detection ranges. There are three broad atmospheric windows in the millimeter range, centered around 94, 140 and 270 GHz, which can be used for remote measurements without much atmospheric attenuation and interference.
Millimeter-wave spectroscopy is an established technique for determining the structure and dynamics of molecules in the gas or vapor phase. Polar molecules selectively absorb electromagnetic radiation of specific wavelengths in the millimeter and submillimeter-regions in accordance with their rotational energy transitions. Of all known analytical techniques, the millimeter-wave technique offers the highest possible spectral resolution if the analyte can be sampled and fed into a gas cell under low pressure. For example, typical line widths (half width at half maximum) of rotational lines are 4 kHz at one millitorr, and in a band width of 1 GHz, 250,000 resolution lines are available.
Millimeter-wave radiometers have been used by researchers for radioastronomical detection of molecules in interstellar clouds and for atmospheric sounding of temperature and density profiles using oxygen and water vapor molecules. See M. A. Janssen (Ed.), "Atmospheric Remote Sensing by Microwave Radiometry," John Wiley, New York (1993). Also a ground-based millimeter-wave sensor based on multispectral radiometry has been successfully employed to measure spectroscopic trace constituents such as HCN, ClO, and HO.sub.2. See A. Parrish et al., "A Ground Based Technique for Millimeter-wave Spectroscopic Observations of Stratospheric Trace Constituents," Radio Science, Vol. 23, pp. 106-118 (1984). Concentrations of gases lower than parts per billion have been measured using this technique. Recently, millimeter-wave multispectral radiometers have been used in the NASA's upper atmospheric research satellite (UARS) and have provided the first global-scale map of chlorine monoxide in the lower stratosphere.
Under low pressures, the technique offers very high spectral specificity. But remote monitoring of chemicals in the air requires measurements to be made at ambient pressure (1 atm). The effect of increased pressure on the rotational line is twofold: one, it reduces the detection peak and two, it broadens the spectral line. As a result, the detection sensitivity and chemical selectivity suffer at high pressures. But these problems can be mitigated by using high frequency millimeter-waves with wide bandwidth. The detection sensitivity increases in general with the square of the frequency and the number of resolution lines (chemical selectivity) increases with wide bandwidths that are practical with high frequency millimeter-waves.
Significant advances have been made in microwave/millimeter-wave systems with applications to defense/space technology, meteorology, and remote sensing of the Earth. Combining the latest developments in high frequency sources in the millimeter and submillimeter-wave range, wide-sweep capability of the sources, and sensitive solid-state millimeter/submillimeter-wave detectors, this invention presents new millimeter/submillimeter-wave sensor approaches that can be successfully applied to remote or inplant spectroscopy at ambient pressures. Both active systems based on swept-frequency radar technique and passive systems based on multispectral radiometer techniques are presented.